1. Field of the Invention
This invention generally relates to a multi-processor system and more particularly to a multi-processor system with programmable memory-access priority control means.
2. Description of the Prior Art
In a multi-processor system, it is very important that a common shared memory or a common bus connected with processors is effectively used.
In the conventional multi-processor system, the memory access operation was generally conducted in response to the priority assigned to each processor under the control of fixed hardware.
The prior art multi-processor system shown in FIG. 1, for example, includes hardware logic circuits in a memory control unit 1 (MCU) to determine the priority of memory-access operation to memory 10 in accordance with requests from processors 11 (CPU.sub.1), 12 (CPU.sub.2) and 13 (CPU.sub.3).
This system has a simple hardware configuration, however, the priority of the processors is fixed by the logic circuits, so that the following inconveniences exist.
Thus, for example, even if processor 11 (CPU.sub.1) executes a dummy loop which is not of an urgent nature and processor 12 (CPU.sub.2) executes a routine which is of a very high degree of urgency and is caused by a request from a direct memory access controller (not shown), then under the superior fixed priority of processor 11 (CPU.sub.1) to processor 12 (CPU.sub.2), the processor 12 (CPU.sub.2) must always wait for the completion of the memory operation by processor 11 (CPU.sub.1). Also, while the dynamic programming in the multi-processor system coordinates each processor so that the efficiency of the system may be improved and the priority of jobs executed by the processors are varied from one moment to the next, the priority of the memory access operation should be designed so as to meet the demands of each need.
Alternatively, a specified supervisory control program may be adopted to assign different programs to a respective processor for execution. But once again, priority of access of a common shared memory by the processors is fixed by the hardware.